Method of atomizing a coal/water suspension and system therefor

ABSTRACT

A coal/water suspension, contained or prepared in a storage vessel, is heated to a temperature corresponding approximately to the boiling point of at least one component of the mixture in the firing chamber of a boiler or the like and is then delivered to this chamber through an atomizing nozzle at a higher pressure at which the suspension is non-boiling.

United States Patent Scheubel 1451 Aug. 8, 1972 54] METHOD OF ATOMIZING A [56] 1161mm cm ggg N AND UNITED STATES PATENTS 72 I t Franz 01181150115 et al. ..1 R I 1 or mm, Gama; 2,747,552 5/1956 Kyrklund 122/7 R 3,229,651 1/1966 Wasp ..110/7 R [731 'F 3,357,375 12/1967 Brophy ..110/7 s mm m 3,447,494 6/1969 Schwartz =1 al ..1 10/28 R [22] Filed: Oct. 8, 1970 21 A L N a 79,010 Primary Examiner-Kenneth W. Sprague I 1 pp 0 Attorney-Karl F. Ross [30] Foreign Application Priority D818 57 I ABSTRACT Oct. 10, 1969 Germany ..P 19 51 172.9 A coal/water suspension contained or prepared in a storage vessel, is heated to a temperature correspond- {Inf-(g1. ..110/7 toll/1012 g approximately to the boiling point of at least one 581 Field 61 Searc 1 1 116/77 28 R 1 P- in firing chambe' a 125/7 boiler or the like and is then delivered to this chamber through an atomizing noule at a higher pressure at which the suspension is non-boiling.

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ing compound H o low-boil- STEAM INVENTOR. Franz N. Scheubel 1 Attorney METHOD OF ATOMIZIN G A COAL/WATER SUSPENSION AND SYSTEM THEREFOR FIELD OF THE INVENTION My present invention relates to a method of atomizrng coal/water suspensions and to a method of improving the combustion of such suspensions in water boilers or the like.

BACKGROUND OF THE INVENTION For some time, techniques in the firing of water heaters, steam boilers and the like with solid fuels have concentrated upon the use of finely-divided solid fuels in place of the automatic stoker and the like system previously serving to feed the furnace chamber. In some systems, steam-producing boilers, for large-scale domestic heating, industrial applications and power generation, have made use of solid-fuel, e.g. coal, in finely divided form, i.e. as a dust. For the most part the dust was simple to displace, but offered various handling difficulties. Attention was then directed to socalled suspension or slurry systems in which a mixture of the solid fuel, again generally coal, and water, wherein the solids predominated, was pumped to the furnace. Numerous advantages were thereby attained.

For example the specific power cost was reduced up to about percent, the wet handling of the finely divided solids simplified the procedure and increased safety and eliminated health difficulties. Furthermore, the conveyance or transport of the coal/water mixture to the furnace chamber was greatly simplified and could be carried out with automatic controls. An important advantage also resided in the fact that coal/water suspensions of the character described, i.e. wherein coal predominates and may make up to 60 percent of the suspension so that the weight ratio of coal to water is 60:40, are highly stable and can be stored for relatively long periods without substantial settling or agglomeration.

The pumps and conduits used to deliver the suspension to the furnace may be of simple construction and,

generally, are less prone to wear than systems using coal dust in the absence of a liquid vehicle. Since the concentration of the fuel in the suspension is relatively high, the distributing pipes for delivering the suspension to the several burners of a particular furnace may be of relatively small size and low velocities may be used. Furthermore, since there is moisture present in the fire chamber, the system can be operated with improved heat efficiency, lower temperatures and reduced soiling of the heating surfaces of, for example, a tube-type boiler. Finally, but not least, the coal/water suspension enables the number of personnel supervising an operating system to be reduced. Hence it is found to be advantageous to burn coal/water suspensions in industrial and other boilers for production of steam and the like. However, the atomization of such suspensions has been a problem heretofore. Under one proposal, the coal/water suspension was forced through an atomizing nozzle into the furnace chamber and, upon formation of the spray at the mouth of the nozzle, was subjected to combustion. The forcing of the suspension through the nozzle is effected by a pump. This system has the disadvantage that the nozzle discharge tends to agglomerate and is neither of the uniformity required, nor of a constant particle size, so

that burning is inefficient and combustion is difficult. l-leretofore, such disadvantages have been associated with the relatively high viscosity of the suspension and have been assumed to be inherent in its use. To avoid these disadvantages, therefore, it has been proposed to use systems which have been found to be effective in some high-viscosity techniques. Such systems included, for example, a rotating cup atomizer with the slurry or suspension being dispensed by centrifugal force over the edge of the cup. While the atomizing uniformity of this arrangement has been found in some ways to be an improvement by comparison with the earlier nozzle system, it is observed that the edge of the cup wears or erodes in a rapid and undesirable manner and an efficient atomization can only be achieved with low suspension throughput and high cup speed. It is not always possible to obtain satisfactory speeds with such systems.

OBJECTS OF THE INVENTION It is, therefore, the principal object of the present invention to provide an improved method of atomizing and/or burning a coal/water suspension wherein the aforementioned disadvantages are obviated.

It is another object of this invention to provide a method of the character set forth which will enable the atomization of coal/water suspensions having weight ratios of coal/dust and water of 60:40 (approximately) without significant erosion of the atomizing means and without the need for high energy.

Another object of my invention is to provide an improved method of atomizing a coal/water suspension so that the dispersion entering the combustion chamber of a boiler or like installation is more uniform and more efficiently combustible than heretofore.

SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are obtained in accordance with the present invention wherein a system or method of atomizing a coal/water suspension in which coal and water are present in a weight ratio of approximately 60:40 and which is based upon my discovery that when the suspension is heated to a temperature at the atomizing pressure which is less than the boiling point of any component of the suspension but is greater than the boiling point of the suspension or a component thereof,

at the pressure sustained within the combustion chamber into which the disbursion is ejected, a highly uniform dispersion without agglomeration occurs. It would appear that the invention is based upon the substantially instantaneous volatilization of the liquid phase or a portion thereof, from the spray or atomized mist as soon as the suspension is introduced into the furnace chamber and the assist given by such impulsive volatilization to a distribution of the solid phase.

More specifically, the invention relates to a process for atomizing and burning a coal/water suspension in which the weight ratio of coal/water is 60:40, i.e. the coal makes up 60 percent by weight (approximately) of the suspension. According to the invention, the suspension is delivered through an atomizing nozzle under an atomizing pressure well above atmospheric pressure to the furnace chamber which preferably is held at atmospheric pressure and is, prior to introduction into the chamber and passage through the nozzle, heated to a temperature corresponding at least to the boiling point of the liquid phase within the furnace chamber. All or part of the heating of the coal/water suspension can be carried out prior to pressurization although it is preferred to heat the suspension subsequent to pressurization as will be apparent hereinafter. I have found, surprisingly, that a similar effect can be obtained when one or more low-boiling point liquids are admixed with the aqueous phase while the low-boiling point liquids can be hydrocarbons having a boiling point below that of water. I prefer to make use of substances, e.g. al cohols which are miscible with water and which may nilake up, say, 1 to 20 percent by weight of the aqueous p ase.

When all of the heating or the major part of the heating is to be carried out prior to displacement of the suspension by pump means or the like, it may be effected in a mixing vessel in which the coal/water suspension is prepared. In this case, the mixing vessel may be held under pressure. Of course, heating means may be provided anywhere along the path of this vessel, i.e. ahead or behind the pump and the heating system may employ a heat exchanger of any conventional configuration or mode of operation. It has been found to be advantageous to control the atomization of the coal/water suspension by adjustment of the heating, it being found that as the heating increases, the atomization efficiency and tendency to form a finely divided mist or dispersion likewise increases Of course excessive heating has the disadvantage that pipelines and other equipment must be able to withstand the correspondingly high pressures.

Among the advantages of the system described above is that it is possible to obtain a finely divided suspension, which is highly uniform and free from agglomeration at relatively low pressures. Furthermore, since the added heat contributes to volatilization of the moisture (liquid phase), the volume of the suspension increases sharply upon its injection into the furnace chamber. The particles of the suspension are. thereby accelerated sharply and any agglomerate which tends to form is immediately broken up. Furthermore, the water vapor contributes to a greater uniformity of the coal particles and the regulation of the nature and degree of atomization can be controlled with ease.

DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing and specific Examples. In the drawing, the sole FIGURE is a diagram illustrating the system of the present invention.

SPECIFIC DESCRIPTION The drawing shows a water-tube boiler located above and within a furnace chamber 11 and having heating surfaces 12 through which heat is transferred to water to convert it to steam in accordance with conventional principles. A water-tube boiler of this type is fully described at pages 9 39 ff. of Pe'rrys Chemical Engineers Handbook, McGraw Hill Book Co. 1963. The air is supplied to the system by any conventional atomizing burner as described at pages 9 26 if. of this publication and represented at 13 in the drawing. The nozzle 13 is supplied via line 14 at an atomizing pressure of 1.2 to 10 atmospheres (absolute) by a slurry pump 15 drawing the coal/water suspension from a mixing vessel 16. Coal, delivered by the pulverizer represented at page 9 of Perrys Chemical Engineers Handbook is supplied in an amount of about 60 percent by weight of the total suspension as represented at 17 while water is supplied at 18. A mixer 19 can be provided to maintain the homogeneity of the suspension. Vessel 16 is of the pressure-retentive type and is shown to be provided with a heating element 20 which may, of course, represent any heating means including gas-fired, oil-fired, waste-heat recovery exchangers or electrical heating means. The heating means 20 is controlled at 21 in accordance with the pressure within the fire box 11 and sensed by a pressure transducer 22 and the pressure within duct 14 as sensed by the transducer 23. When all or part of the heating is to be carried out ahead of the pump 15, a further heat exchanger 24 may be provided along line 14.

The suspension is prepared under pressure in receptacle 16 and is there heated to a temperature just below the boiling point of the liquid vehicle of the suspension within this vessel 16. The suspension is then displaced by pump 15 to the nozzle 13 so that its atomizing pressure within line 14 exceeds the pressure in vessel 16, the additional heater 24 being provided to further increase the temperature of the suspension in line 14 to a level just below its boiling point and well above the boiling point at the liquid phase at the pressure prevalent in the fire box 11. The discharge from the nozzle 13 into the fire box is invariably found to be of high uniformity and it has been found to be possible, using this technique, to reduce the atomizing pressure necessary for equivalent atomizing efficiencies.

Line 26 delivers from the boiler 10, 12, the steam necessary to heat the suspension via the heating coils 20 or 24. Of course, the steam may be injected directly into the suspension.

SPECIFIC EXAMPLES Example I An industrial furnace is fired through an atomizing nozzle of the character described with 400 kg/hour of pulverized coal (particle size below 200 mesh) formed into a 60:40 weight-ratio suspension with water and delivered by the pump 15 at an atomizing pressure of four atmospheres (absolute). The suspension is heated prior to emergence into the furnace chamber 11 which is held at atmospheric pressure, to a temperature of C. A highly efficient combustion and dispersion was obtained. Attempts to obtain such a dispersion without heating of the suspension by increasing the pressure in line 14 failed when six atmospheres was reached and could not be exceeded because of the nature of the equipment.

Example II The system of the drawing and of Example I was used except that 15 percent by weight methanol was substituted for an equivalent portion of the aqueous phase. It was found to be possible to heat the suspension to only 120 C to obtain a result. approximately equivalent to that set forth in Example I.

I claim:

1. A method of atomizing a coal/liquid suspension consisting of pulverized coal and a liquid for combustion in a furnace chamber, comprising the steps of:

forcing said suspension under a superatmospheric atomizing pressure through an atomizing nozzle and into said chamber;

heating said suspension prior to its emergence from said nozzle to a temperature above the boiling point of the liquid at the pressure in said chamber;

the step of regulating the heating of said suspension to adjust the atomization thereof at said nozzle.

6. The method defined in claim 2 wherein said suspension is heated by storing said suspension in a pressure-retaining vessel and heating the suspension within said vessel.

7. The method defined in claim 2 wherein said suspension is pumped to said nozzle, the heating of the suspension being carried out between the pump and said nonle.

8. The method defined in claim 2 wherein said suspension consists substantially of 60 parts by weight coal and 40 parts by weight water.

9. A combustion apparatus comprising a furnace chamber, an atomizing nozzle opening into said chamber, means for forming a coal/water suspension, a pump for dispensing said coal/water suspension under an atomizing pressure to and through said nozzle, a pressure-retentive nozzle, for delivering said suspension to said pump, heating means at said vessel for heating said suspension, and means for introducing at least one low-boiling compound into said suspension prior to its passage to said nozzle. 

2. The method defined in claim 1 wherein said liquid consists predominantly of water and said suspension contains about 60 percent by weight coal.
 3. The method defined in claim 2 wherein said suspension is heated to a temperature below the boiling point of the liquid at said atomizing pressure.
 4. The method defined in claim 2 wherein said suspension is heated by steam.
 5. The method defined in claim 2, further comprising the step of regulating the heating of said suspension to adjust the atomization thereof at said nozzle.
 6. The method defined in claim 2 wherein said suspension is heated by storing said suspension in a pressure-retaining vessel and heating the suspension within said vessel.
 7. The method defined in claim 2 wherein said suspension is pumped to said nozzle, the heating of the suspension being carried out between the pump and said nozzle.
 8. The method defined in claim 2 wherein said suspension consists substantially of 60 parts by weight coal and 40 parts by weight water.
 9. A combustion apparatus comprising a furnace chamber, an atomizing nozzle opening into said chamber, means for forming a coal/water suspension, a pump for dispensing said coal/water suspension under an atomizing pressure to and through said nozzle, a pressure-retentive nozzle, for delivering said suspension to said pump, heating means at saId vessel for heating said suspension, and means for introducing at least one low-boiling compound into said suspension prior to its passage to said nozzle. 